1. Field of the Invention
The present invention relates to a method of canceling interference components included in received signals of a base station in a mobile communication system, and more particularly, to a method of canceling interference components included in received signals of a base station in a mobile communication system, which allows user data not to serve as interference components for other user data by subtracting the user data from received signals which are standby for decoding the other user data.
2. Background of the Related Art
Generally, in a mobile communication system based on code division multiple access (CDMA) type, respective mobile terminals transmit transmission data by spreading with its own unique PN sequences so that the data can be transmitted to the closest base station with the same radio frequency shared by other users. That is to say, communication between the base station and a plurality of terminals will be performed with the same radio channel. In the mobile communication system based on CDMA type, the plurality of terminals in the same base station area transmit scrambled data to the base station using the same radio channel. Accordingly, a receiving portion of the base station converts received signals including a plurality of user data into base band signals when the plurality of terminals in the service area of base station receive the user data. The receiving portion despreads the base band signals using the unique PN sequences of the respective terminals to identify, which have been known by the base station, and decodes them to decode them to original data. The process is limited by the interference generated by the other transmitters.
FIG. 1 is a block diagram illustrating a partial configuration of a typical base station receiving portion.
Referring to FIG. 1, radio signals transmitted from respective terminals are input to a frequency down processor 10 of the base station through an antenna of the base station and then converted into base band signals. Such base band signals are in parallel input to a plurality of multipliers 21axcx9c21n of a composite correlation bank 20 so that the base band signals are respectively multiplied by the unique PN sequences C1ejo1, C2ejo2, . . . , CNejoN of the respective terminals, which have been already known by the base station. Subsequently, spreading values output from the respective multipliers 21axcx9c21n are respectively input to a plurality of accumulators 22axcx9c22n which are connected to the multipliers 21axcx9c21n in series, and then stored as correlation values corresponding to the respective user data for the unit of symbol(or frame).
The respective accumulators 22axcx9c22n transmit the user data stored for the unit of symbol(or frame) to the next circuit to decode them to original signals.
The base station receiving portion of FIG. 1 is supposed on the assumption that all the user data transmitted from the respective terminals have the same transmission rate. In other words, it is noted that such a base station receiving portion is supposed taking account of only mobile communication system which support only voice service. Accordingly, such a base station receiving portion has simple hardware configuration and processes data at relatively high speed because it detects user signals including interference signals. On the other hand, since the base station receiving portion of FIG. 1 detects the user signals without canceling the interference components, it has a problem that receiving performance is poor, thereby reducing traffic capacity of the overall system.
To solve such a problem, there is suggested a related art base station receiving portion disclosed in xe2x80x9cIEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS (VOL. 12, NO. 5, JUNE 1994)xe2x80x9d, as shown in FIG. 2.
Referring to FIG. 2, data transmitted from respective terminals in a corresponding base station are input to a frequency down processor 100 through an antenna of the base station and converted into base band signals. Subsequently, the base band signals output from the frequency down processor 100 are stored in a memory 200 for the unit of symbol (or frame). The base band signals stored in the memory 200 are input to a plurality of multipliers 310axcx9c310n of a composite correlation bank 300 in parallel, so that the base band signals are respectively multiplied by the unique PN sequences C1ejo1, C2ejo2, . . . , CNejoN of the respective terminals, which have been known by the base station, to obtain correlation values. Subsequently, output signals of the respective multipliers 310axcx9c310n are respectively input to a plurality of accumulators 320axcx9c320n which are connected to the multipliers 310axcx9c310n in series, and then stored as correlation values corresponding to the respective user data for the unit of symbol (or frame).
Thereafter, the respective accumulators 320axcx9c320n provides correlation values output from the respective multipliers 310axcx9c310n to a comparator 400 connected to the respective multipliers 310axcx9c310n for the unit of symbol (or frame) in parallel. The comparator 400 compares the correlation values output from the respective accumulators 320axcx9c320n with one another to detect one user data having the maximum value. At this time, the detected maximum correlation value is transmitted to the next circuit to be decoded to original data.
Unlike the base station receiving portion of FIG. 1, the user data detected to be decoded to original data are to be feedback to the memory 200 and subtracted from the base band signals including a plurality of user data, so that the detected user data do not serve as interference components for other user data during the next despreading step.
In other words, the correlation value having the maximum value, which is output from the comparator 400, is also input to a multiplier 500 so that the correlation value is multiplied by the unique PN sequences C1ejo1, C2ejo2, . . . , CNejoN of the terminal used for spreading. Signals spread again through the multiplier 500 are provided to a subtracter 600. The subtracter 600 subtracts only the same spread signal as the input spread signal from the base band signals stored in the memory 200. Accordingly, the decoded user data are fully canceled from the base band signals stored in the memory 200.
Subsequently, the base band signals stored in the memory 200 are again input to the plurality of multipliers 310axcx9c-310n of the composite correlation bank 300 in parallel, so that the base band signals are respectively multiplied by the unique PN sequences C1ejo1, C2ejo2, . . . , CNejoN of the respective terminals to obtain correlation values having next priority. The output signals of the respective multipliers 310axcx9c310n are respectively input to the accumulators 320axcx9c320n which are connected to the multipliers 310axcx9c310n in series, and then stored as correlation values corresponding to the respective user data for the unit of symbol (or frame).
Then, the respective accumulators 320axcx9c320n provides correlation values output from the respective multipliers 310axcx9c310n to the comparator 400 connected to the respective multipliers 310axcx9c310n for the unit of symbol (or frame) . The comparator 400 compares the correlation values output from the respective accumulators 320axcx9c320n with one another to detect one user data having the maximum value. At this time, the detected maximum correlation value is transmitted to the next circuit to be decoded to original data.
As aforementioned, the base station receiving portion of FIG. 2 performs despreading and decoding operations in the order of the user data having the maximum correlation value until there exists no user data in the memory 200, and subtracts only the same spread signal as input spread signal from the base band signals stored in the memory 200 in the order of detected user data to sequentially cancel already decoded user data.
However, such a related art base station receiving portion has several problems.
Since the aforementioned operations should be performed repeatedly until the plurality of user data included in one frame are sequentially despread and decoded to be sequentially canceled, counting time and counting amount for despreading and decoding the received signals of the base station and canceling the despread and decoded user data from the memory have serious deviation depending on the number of the current users. Also, since the receiving portion of FIG. 2 is envisioned on the assumption that the current users have the same data transmission rate, it is undesirable for multimedia services supporting various transmission rates.
Furthermore, it is difficult for the related art base station receiving portion to reduce the number of correlators used for despread of received base band signals. This goes back to the trend to small sized communication systems which improve complexity in hardware.
Accordingly, the present invention is directed to a method of canceling interference components included in received signals in a base station of a mobile communication system, that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method of canceling interference components included in received signals of a base station in a mobile communication system, which despreads user data in the order of user data grouped for the unit of transmission rate and sequentially cancels user data which are the same as despread user data by allowing the base station to detect transmission rate and user codes of user data transmitted from respective terminals, in case that the respective terminals transmit various user data which require various transmission rates in the mobile communication system of CDMA type.
Other object of the present invention is to provide a method of canceling interference components included in received signals of a base station in a mobile communication system, which minimizes the number of correlators for despreading received signals of the base station to reduce counting time and counting amount for despread and reduce complexity in hardware.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method of canceling interference components included in received signals of a base station in a mobile communication system is characterized in that the base station receives user data transmitted from respective terminals in its own area to sequentially perform grouping of the user data in the order of high transmission rate using transmission rate of the respective user data and user codes of the respective terminals, which are in advance known. Subsequently, the grouped user data are correlated in parallel to coincide with the number of correlators which support the system. Then, the correlated resultant values are compared with one another to decode one correlation value having the maximum value, and one correlation value having the maximum value is spread using the user codes, so that user data obtained during spreading step are subtracted from input user data.
In another aspect, a method of canceling interference components included in received signals of a base station in a mobile communication system is characterized in that the base station receives user data transmitted from respective terminals in its own area to perform grouping of a plurality of user data depending on already known transmission rate of the data. Subsequently, the user data are correlated in parallel for the unit of group using user codes of the respective terminals, which have been known by the base station, and then decoded to be decoded to original data. Then, the correlated resultant values are spread in parallel for the unit of group using the user codes, and a plurality of user data obtained during spreading step are canceled from input user data for the unit of group.
In other aspect, an apparatus for canceling interference components included in received signals in a base station of a mobile communication system, includes a memory for storing user data transmitted from at least one or more terminals for the unit of symbol(or frame), a central processing unit (CPU) for knowing both transmission rate of data to be transmitted and user codes of respective terminals if the respective terminals perform communication, to select user data included the data to be transmitted with at least one or more groups depending on transmission rate, a correlation portion for correlating user data pertaining to a group having the highest priority among the groups selected using the user codes, and a spreading portion for spreading the correlated resultant values using the user codes and providing user data obtained during spreading step to the memory in parallel to sequentially subtract the user data stored in the memory.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.